The present invention is generally directed towards a green glass used as automotive or architectural glass. More particularly, to a soda-lime-silica glass having improved UV absorption and low to medium visible light transmittance.
As is well known in the art, iron oxide is commonly used to provide a green color to glass. In the glass, the iron oxide exists in two chemical forms, an oxidized form: Fe2O3 wherein the iron is Fe+3 and a reduced form: FeO wherein the iron is Fe+2. Advantageously, the oxidized form of iron oxide absorbs a portion of the ultra violet (UV) light passing through the glass product and the reduced form absorbs a portion of the infrared (IR) light passing through the glass product. As would be appreciated, the UV and IR light absorption properties of iron oxide are especially valuable when the glass is used in automobiles. When heat is absorbed by the glass, the load on air conditioners is initially reduced and there is less total heat in the vehicle to cool. When the ultra violet absorption is improved, there is less damage over time to the colors of the components inside the vehicle and provides for more passenger comfort. Therefore, controlling these spectral properties of the glass is very important.
Under composition batching and furnace firing conditions generally used in the glass industry, if the total iron oxide as Fe2O3 in the glass composition is within about 0.3 to 2.0 wt. %, the iron oxide equilibrium provides a Fe+2/Fe+3 weight ratio greater than 0.35. Adding iron oxide to the glass under normal furnace conditions improves both the UV and the infrared absorption of the glass since the concentration of the iron forms is correspondingly increased, but this improvement is at the expense of visible transmittance. That is, as iron oxide is added the color of the glass darkens so that the visible transmittance is correspondingly decreased.
It would be extremely advantageous to improve the UV absorption of green glass products while maintaining a high level of visible transmission and to also have a good absorption in the IR portion of the spectrum. These advantages could possibly be obtained by including more iron oxide in the glass composition while providing a more oxidizing environment in the glass furnace to shift the iron oxide towards its oxidized form. This would increase the UV absorption of the glass. And by shifting the iron oxide away from its darker reduced form towards the oxidized form, even more iron oxide perhaps could be added to the batch to further improve UV and IR light absorption. All of this could possibly be done while still maintaining good visible transmittance properties of the glass.
One way commonly employed to shift towards a more oxidizing environment in the glass furnace is by providing additional air to the glass melt in the furnace. Increasing the amount of air, however, has several undesirable consequences: the furnace cools down, the combustion heating of the furnace becomes inefficient which requires a fuel gas increase and also the increased oxygen can promote the formation of undesirable NOx emissions in the combustion products.
Sodium sulfate, a fining agent often added to the glass melt to remove bubbles from the glass, can also act as an oxidizing agent. Increasing the sodium sulfate in the glass batch in an amount to effectively oxidize the batch, however, is also less than desirable. Excess sodium sulfate can generate undesirable SOx emissions once the saturation point of solubility of sulfate in the glass melt is reached. Anthracite coal (a reductant) is another material typically used in glass melts along with sodium sulfate. It causes sodium sulfate to break down into sodium oxide which becomes part of the glass and sulfur trioxide which generates the fining action to remove bubbles in the melt. The glass batch can be made oxidizing by simply removing the coal from the batch, but then the break down of the sodium sulfate requires that the temperature in the furnace be raised which makes for less efficient furnace operation. Generally, increasing the quantity of sodium sulfate in the glass tends to shift the iron oxide equilibrium toward oxidizing while increasing carbon concentration in the glass batch shifts the iron oxide equilibrium toward reducing. Furnace temperature also affects iron oxide equilibrium. Increased temperature shifts the iron oxide toward the reduced state and lowering overall furnace temperature allows the iron oxide to move toward the oxidized state. Generally lowering furnace temperatures, however, can potentially lead to defects in the final glass product.
An often used and well known oxidizing material, sodium nitrate, can also be added to the glass batch to shift the iron oxide towards its oxidized form. It is only effective, however, as an oxidizer in the early stages of glass melting which limits control of the iron oxide redox equilibrium. Another more negative aspect of using sodium nitrate is that environmentally undesirable nitrogen oxide emissions are generated. Thus, attempting to shift the redox equilibrium toward the oxidized iron oxide form by using sodium nitrate is less than satisfactory for several reasons.
As it can be clearly appreciated from the above discussions, there are significant difficulties associated with providing and maintaining particular oxidizing conditions in a glass melt furnace to control the redox ratio of the iron oxide. To avoid these difficulties, UV absorbing materials like oxides of cerium, titanium, vanadium, and chromium are often added to the glass batch to increase the UV absorption of the glass. When used, however, they are included in the glass in very small quantities, which only provide limited UV improvement. One reason for using only small quantities is that some of these additives are very expensive. Cerium oxide, in particular, can more than double the batch cost when used in a sufficient quantity to adequately improve the ultra violet absorption of glass products. Titanium dioxide is less expensive than cerium oxide but is still much more expensive than iron oxide. Chromium oxide must also be used in extremely small quantities because while the oxidized form of chromium oxide absorbs in the ultra violet portion of the spectrum, the reduced form of chromium absorbs in the visible portion of the spectrum causing a loss of the visible transmission and much stronger color in the glass product. Vanadium oxide also has been known to deteriorate furnace refractories. Therefore the improvement in UV absorption that can be obtained by using such additives is commercially limited.
In the present invention a unique and commercially desirable way to improve the UV and the IR absorbing properties of a green glass composition while maintaining desirable visible light transmittance is disclosed. In accordance with the teachings of the preferred embodiment a manganese compound like manganese dioxide in the glass melt along with the iron oxide is added. Manganese dioxide is found to be an excellent and inexpensive oxidizer of the glass melt which avoids the drawbacks of prior art oxidizers like sodium nitrate.
In another aspect of the preset invention, the green glass tint is obtained by eliminating the use of chromium. The present invention is a green soda-lime-silica glass composition that is heat absorbing and has improved ultra violet light absorption, including a manganese compound in the glass melt forces the glass composition batch towards strongly oxidizing furnace conditions so that the iron oxide in the melt is directed toward its oxidized form. Glass products made according to embodiments of the invention have the following spectral properties at 4.0 mm. thickness: 15 to 45% light transmittance using Illuminant A (LTA) and less than 13% ultra violet (UV) transmittance measured over the range of 300 to 400 nanometers. Generally, as the quantities of the colorants increase, both the % LTA and % UV trasmittance will go down. Similarly, as the glass thickness increases for a given glass composition, the transmittance of the thicker glass decreases. Preferably, the dominant wavelength at a glass thickness of 4 mm is between 494 to 563 nanometers.
According to another aspect of the invention, it is a method for improving the ultraviolet light transmittance while maintaining low to medium visible light transmittance of a soda-lime-silica green glass composition using iron oxide as a colorant by including a manganese compound along with the iron oxide during melt processing of the glass composition. The method comprises admixing components which result in the glass composition disclosed above.
These and still other advantages of the present invention will become apparent from the detailed description.